Method of manufacturing a dual gate semiconductor device with a poly-metal electrode

ABSTRACT

In order to realize a dual gate CMOS semiconductor device with little leakage of boron that makes it possible to divisionally doping a p-type impurity and an n-type impurity into a polycrystalline silicon layer with one mask, a gate electrode has a high melting point metal/metallic nitride barrier/polycrystalline silicon structure. The boron is pre-doped in the polycrystalline silicon layer. The phosphorus or arsenic is doped in an n-channel area. Then, the annealing in a hydrogen atmosphere with vapor added therein is performed. As a result, the boron is segregated on the interface of the metallic nitride film and the phosphorus is segregated on the interface of the gate oxide film, for forming an n+ gate.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a CMOS (ComplementaryMetal-oxide Semiconductor) type semiconductor device having as a gateelectrode p type and n type conductive polycrystalline silicon and amethod of manufacture thereof.

[0002] The current trend of the semiconductor device has been changedfrom a CMOS semiconductor device whose gate electrode is composed ofonly polycrystalline silicon containing an n-type impurity into theso-called dual gate CMOS semiconductor device in which the gateelectrode of an n-channel MOS transistor is composed of polycrystallinesilicon containing an n-type impurity and the gate electrode of ap-channel MOS transistor is composed of polycrystalline siliconcontaining a p-type impurity. This dual gate contains a silicon oxidefilm, a silicon nitride film or resist coated on the surface of thepolycrystalline silicon except a target area. A target area (forexample, n-type) impurity is implanted in the non-coated area with thesefilms as a mask by means of the ion-implantation technique or thediffusion technique. After removing this mask materials, the surface ofthe polycrystalline silicon on the area having the above-mentionedimpurity is covered with the similar mask materials. The impurity havingthe opposite conductive type (for example, p type) to theabove-mentioned impurity is implanted in the non-covered area by meansof the ion-implantation technique.

[0003] For dividing the polycrystalline area into an n+ and a p+conductive areas, as mentioned above, a mask material is formed on aspecific area in doping a target impurity, which needs two associativeprocesses of forming a mask. It is thus necessary to perform eachprocess of forming a mask film, doing photolithography, and dry-etchinga mask film twice. It means that the dual gate CMOS semiconductorcomposed as above has more manufacturing steps than the CMOSsemiconductor device composed of polycrystalline silicon containing onlyone conductive impurity. This brings about the lowering of amanufacturing yield and the rise of a manufacturing cost of thesemiconductor device and therefore the rise of a product cost. Further,the slip of fitting the mask patterns may be brought about in dividingthe n-type and the p-type areas in the photolithography process. Hence,the fitting allowance is required, which has been an obstacle to finningthe element and enhancing the concentration of the semiconductor device.

[0004] On the other hand, the MOS transistor having as a component of agate electrode polycrystalline silicon or amorphous silicon filmcontaining boron has a shortcoming that by performing a high temperatureannealing with respect to the MOS transistor after forming the gateelectrode, the boron is diffused from the gate electrode into a gateoxide film, in a worse case, penetrates the gate oxide film and reachesthe silicon substrate, thereby causing a threshold voltage of thetransistor to shift from a design value. It has been reported that theannealing in a hydrogen atmosphere diffuses boron through the gate oxidefilm faster than the annealing in a nitrogen atmosphere. (IEEE ElectronDevice Let., Vol.17, No.11, pp497 to 499) In order to enhance theintegration of the semiconductor device, the need for development of anSAC (Self-aligned contact) technique has risen. This SAC technique iscomposed by covering the gate electrode with a silicon nitride film anda silicon oxide serving as an insulating film between a gate electrodeand the upper metal layer for an interconnection on the silicon nitride.Then, a contact hole, which is served as connecting the source and thedrain of the MOS type semiconductor device with the electrode wiring, isformed by the dry-etching technique. In this dry-etching technique, theprocess is conditioned in order to make the etching speed of the siliconoxide faster than that of the silicon nitride. As a result, if thedeviation of the designed position of a contact hole to a gate electrodepattern is brought about in the photolithography process, no short takesplace between the gate electrode and the source or the drain.

[0005] The silicon nitride film that is important to this SAC techniqueis generally formed by means of the CVD (Chemical Vapor Deposition).This silicon nitride film contains several percents to 20 percents ofhydrogen. This hydrogen serves to accelerate leakage of boron containedin the polycrystalline silicon layer into the substrate. In an extremecase, the amount of boron to be leaked through the gate oxide film ismade so large that the channel area n-type silicon substrate of thep-channel MOS transistor is inverted into the p-type one.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide a dual gateCMOS type semiconductor device which is composed to simplify the processof doping an impurity into a polycrystalline silicon layer formed in theaforementioned dual gate CMOS, reduce the leakage of boron in thehydrogen-related process as much as possible, and thereby have a targetthreshold voltage.

[0007] In the MOS transistor having as a gate electrode an amorphous orpolycrystalline silicon film containing boron, by performing a hightemperature annealing in a hydrogen atmosphere for the transistorfabrication (for example, at a temperature of 950° C. and for tenminutes), the threshold voltage Vth obtained in the case of performingan annealing in a nitrogen atmosphere at the same temperature and duringthe same interval is shifted from about 1.5 V to 2.0 V. This indicatesthat the accelerating effect of hydrogen on boron diffusion in a siliconoxide film may bring about leakage of boron from the gate electrode intothe substrate.

[0008] It has been found by means of the secondary ion mass spectroscope(SIMS) that if the annealing in a nitrogen atmosphere is performed withrespect to the surface of the polycrystalline silicon containing boron,coated with a metal nitride film such as tungsten nitride (WNx) andtitanium nitride (TiNx), the boron doped in the polycrystalline siliconis segregated into the interface of the metal nitride film as shown inFIG. 2A. Even in the same structure, however, the annealing in ahydrogen atmosphere brings about diffusion of boron onto the substrate,thereby causing lots of boron to be leaked into the substrate throughthe gate oxide film.

[0009] If the annealing is performed in an atmosphere where severalpercents of vapor is added to hydrogen, as shown in FIG. 2C, it has beenfound that the boron contained in the silicon is segregated into themetal nitride interface like the annealing in a nitrogen atmosphere.

[0010] On the other hand, it has been found that phosphorus or arsenicused as an n-type impurity doped in polycrystalline silicon issegregated into the gate insulating film interface composed of siliconoxide on the lower layer and has a high concentration on the interface.

[0011] This phenomenon, in which the boron served as a p-type impuritycontained in the polycrystalline silicon in the metal nitridefilm/polycrystalline silicon/gate insulating film structure has adifferent segregated interface from phosphorus or arsenic served as ann-type impurity contained therein, makes contribution to bothsimplifying the dual gate CMOS process and reducing leakage of boron ata time.

[0012] That is, at first, boron is contained on the overall surface ofthe polycrystalline silicon film and a material served as a mask forscreening phosphorus or arsenic is coated on the surface outside of thetarget area. In this state, an n-type impurity such as phosphorus orarsenic is implanted onto the non-mask area by means of theion-implantation technique or the diffusion technique. Then, the maskmaterial is removed and then another layer such as a metallic nitridelayer is formed in place. Next, the annealing is performed in a nitrogenatmosphere or a hydrogen atmosphere with vapor added thereto. As aresult, boron is segregated on the metallic nitride interface in thearea where boron and an n-type impurity are dually doped (for example,in the case of implanting 2×10¹⁵/cm² of boron or arsenic) and theconcentration is made lower to (1×10²⁰/cm²) on the insulating filminterface on the ground. On the other hand, arsenic is segregated ontothe gate insulating film interface and thus is high in concentration(2×10²⁰/cm²). As a result, as shown in FIG. 1, the area in whichphosphorus is contained is made to be an n-type one, while the otherarea includes a polycrystalline silicon gate electrode having a p-typeconductive characteristic. That is, only one process for specifying adoping area makes it possible to form a dual gate and allow the boron inthe polycrystalline silicon to be segregated onto the metallic nitrideinterface, thereby reducing the leakage of boron.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a graph showing a measured result by a secondary ionmass spectrograph with respect to the p-type impurity and the n-typeimpurity in the NMOS of the semiconductor device according to the firstembodiment of the invention;

[0014]FIGS. 2A to 2C are graphs showing boron distributions afterannealing in the NMOS of the semiconductor according to the invention;

[0015]FIGS. 3A to 3E are sectional views of a semiconductor device ateach step of a manufacturing method according to the first embodiment ofthe invention;

[0016]FIGS. 4A to 4E are sectional views of a semiconductor device ateach step of a manufacturing method according to the first embodiment ofthe invention;

[0017]FIG. 5 is a graph showing a comparison between the transistor ofthe invention and the conventional transistor;

[0018]FIGS. 6A to 6B are graphs showing dependency of an impurityconcentration on a depletion width formed in an n+/p+ junction in theNMOS of the semiconductor device according to the invention anddependency of an impurity concentration on a breakdown voltage of thejunction therein; and

[0019]FIG. 7 is a graph showing a measured result by a secondary ionmass spectrograph with respect to a p-type impurity and an n-typeimpurity in the NMOS of the semiconductor device according to the thirdembodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

[0020] The description will be oriented to a process of making a dualgate CMOS transistor according to this embodiment with reference toFIGS. 3A to 3E and 4A to 4E. Herein, the description will be oriented toa stacked gate electrode composed of a metal (tungsten) stacked on themetallic nitride formed on the polycrystalline silicon. This structurealso serves to lower the resistance of the gate electrode. In thesefigures, the left-half portion indicates the NMOS formed area, while theright-half portion indicates the PMOS formed area.

[0021] In the following description, the well-related process forforming a trench type insulating area for separating elements providedon a silicon substrate and a p-channel and an n-channel MOS transistorareas on the same silicon substrate and the process for doping animpurity onto the channel areas for keeping the threshold voltage ofeach MOS transistor at a target value will be roughly described below,because those processes are not essential components to the presentinvention. Moreover, though the actual highly integrated semiconductordevice employs multi-interconnection, the present embodiment does notuse it for the convenience's sake.

[0022] A p-type 10 Ωcm silicon substrate 1 is prepared. A trench of 400nm is formed in the interface between elements to be formed on thesubstrate by means of the photolithography technique and the dry-etchingtechnique. Then, after the 10 nm oxidation on the silicon surface, asilicon oxide film having a depth of 500 nm is formed by means of theCVD technique. Then, by performing an annealing of 1000° C. in an oxygenatmosphere, the silicon oxide film is densitied. Next, silicon oxide isremoved from a flat portion of the silicon substrate by means of the CMP(Chemical Mechanical Polishing) technique and then silicon oxide 2 isburied only in the trench (see FIG. 3A).

[0023] Next, for forming a p-type well area 3 and an n-type well area 4at a predetermined areas where a p-channel and an n-channel MOStransistors are to be formed, boron is ion-implanted into the left-halfportion and phosphorus is ion-implanted into the right-half portion.Then, the annealing of 1000° C. in a nitrogen atmosphere is carried outfor 20 minutes. Afterwards, by oxidizing the surface of the substrate at850° C., an SiO₂ film having a thickness of 2.8 nm is formed. Then, thesurface is heat-treated at an oxide nitrogen atmosphere for forming asilicon oxide nitride film. These annealings complete a gate insulatingfilm 5 of 3 nm (see FIG. 3B). The impurity concentration on the surfaceof the substrate that is approximate to adjusting the threshold voltageof each MOS transistor may be achieved by ion-implanting phosphorus orboron in the range of 1×10¹²/cm² to 3×10¹³/cm². In addition, thoughvarious implantations are tried, even for the dual gate CMOS based onthe conventional method, the manufacturing conditions of the MOStransistor having the most approximate threshold voltage are notdefined.

[0024] Next, amorphous silicon 6 is deposited to a thickness of 70 nm.In forming the amorphous silicon, borohydride gas is introduced fordoping boron into the amorphous film. A certain material was produced byusing the ion-implanting method for doping boron onto the amorphoussilicon film. In this case, the amorphous silicon film to which noimpurity is doped is pre-formed and then boron ions are implanted at avoltage of 5 keV in the range of 1×10¹⁵ units/cm² to 5×10¹⁵ units/cm².In actual, various materials were produced with various implantations.

[0025] Then, a silicon oxide film is deposited at a depth of 200 nm bymeans of the CVD method. Then, a silicon oxide film 7 is left only attarget positions (where the PMOS is formed) on the wafer by means of thephotolithography and the dry-etching technique. Afterwards, with thissilicon oxide film 7 as a mask, phosphorus 8 is ion-implanted into theamorphous silicon film of a non-masked area (where the NMOS is formed).Various amounts of phosphorus were implanted at 15 keV in the range of1×10¹⁵ units/cm² to 5×10¹⁵ units/cm² (see FIG. 3D).

[0026] Next, contaminant adherent to the surface of the material duringthe ion-implantation is removed by the method such as ashing and then anoxide film on the amorphous silicon is removed by means of hydrofluolicacid. Then, a tungsten nitride (WNx) 9 is formed to have a thickness of5 nm at a low pressure atmosphere (0.3 Pa) of a mixture of an argon gasand a nitrogen gas by means of the sputtering equipment. In succession,within the same chamber, by stopping the nitrogen gas flow, at anatmosphere of only Ar gas, the tungsten (W) 10 is deposited at a depthof 50 nm by means of the sputtering technique (see FIG. 3E). Then, asilicon nitride film having a depth of 150 nm is formed by means of theplasma CVD method.

[0027] Then, by using the normal photolithography technique anddry-etching technique, the silicon nitride film 11 located on the toplayer is worked to a target gate electrode form (see FIG. 4A). Then,with the silicon nitride film 11 as the mask, the tungsten, the tungstennitride and the amorphous silicon layer are worked at a batch (see FIG.4B). Afterwards, the photoresist located at the top is removed by theconventional plasma ashing process. Then, the surface oxide of thesilicon substrate around the gate electrode that is contaminated anddamaged by those dry etching is wet-etched by means of a mixture ofhydrofluoric acid and vapor.

[0028] Next, vapor is added to the hydrogen atmosphere at a pressureratio of 10% and then the annealing is performed at 750° C. and for 30minutes. This heat treatment results in oxidizing the silicon substratesurface 12 around the gate electrode and the side wall 13 of the gateelectrode silicon layer under W/WNx that has been changed from anamorphous material into a polycrystalline material. In addition, theannealing at the atmosphere of vapor-added hydrogen results inselectively oxidizing the exposed silicon surface without oxidizing thetungsten 8 and the tungsten nitride 10. This process for selectivelyoxidizing the silicon is intended to recovering reliability of the gateoxide film that was subject to damage in the dry-etching process. Afterthe process of re-oxidizing the silicon substrate, a resist pattern forcovering the PMOS area is formed by means of the normal photolithographytechnique. With the resist pattern and the silicon nitride film 11 onthe NMOS area as mask materials, by using the ion-implanting device,phosphorus ions of 2×10¹³/cm² are implanted onto the silicon substrateat 10 keV for forming an n-type semiconductor area 14. Then, the resistpattern for covering the NMOS area is formed by means of the normalphotolithography technique. With this resist pattern and the siliconnitride film 11 of the pMOS area as the masks, BF2 ions of 2×10¹⁴/cm²are implanted at 10 keV by means of the ion-implanting method, forforming a p-type semiconductor area 15 (see FIG. 4C).

[0029] After the process for implanting ions, the silicon nitride filmis deposited to a thickness of 70 nm by means of the low pressure CVDdevice. Afterwards, the silicon nitride film on the flat portion isremoved by the anisotropic ion etching device, so that the siliconnitride film 16 is left only on the side wall of the gate electrode.Then, in the same procedure of implanting ions, the n-type and p-typeimpurities are implanted onto the silicon substrate. In this case,arsenic ions of 2×10¹⁵/cm² are implanted as an n-type impurity at 40keV, and then BF2 ions of 2×10¹⁵/cm² are implanted as p-type impurity at15 keV, for forming an n+ type semiconductor area 17 and a p+ typesemiconductor area 18 (see FIG. 4D).

[0030] After these series of processes for doping an impurity, theannealing is performed for 30 seconds and at a temperature of 950° C.and in a hydrogen atmosphere with 5% of vapor added thereto. Thisannealing is intended for electrically activating the implanted impurityon the silicon substrate. If this annealing is performed in a nitrogenatmosphere, the hydrogen contained in a silicon nitride film formed onthe side wall of the gate electrode causes boron to be likely leakedfrom the gate electrode into the substrate. After forming the source andthe drain of the MOS transistor, the thin silicon oxide film formed onthe surface of the silicon substrate is removed by the hydrofluoric acidsystem etchant. In succession, Co is deposited to a thickness of 8 nm bymeans of the sputtering device. On the layer of Co is deposited TiNxhaving a depth of 20 nm. In this state, the annealing is performed at atemperature of 550° C. and in a nitrogen atmosphere. Then, with amixture of sulfuric acid and hydrogen peroxide, the layer of TiN and thelayer of Co that is not reactive with the silicon of the substrate areremoved by the wet etching method. This annealing causes a CO silicide19 to be left only on the pre-ion-implanted surface of the siliconsubstrate for the purpose of self-adjustment. Further, the annealing ata temperature of 850° C. and in a nitrogen atmosphere causes the Cosilicide to be low in resistance.

[0031] Next, an SiO₂ film served as an interlaminar insulating film 20is deposited at a depth of 500 nm by means of the plasma CVD method.Then, on the surface of the SiO₂ film is coated fused silica glass.Then, the annealing is performed at a temperature of 950° C. and for oneminute. Afterwards, a contact hole is formed for electrically conductinga given spot by means of the photolithography and the dry etchingtechnique. In the process for forming the contact hole, the connectinghole to the source and the drain is partially overlapped with the gateelectrode in the fine MOS transistor because of the shift of the maskfitting of the photolithography. However, since a relatively thicksilicon nitride film is formed on the gate electrode, in the process forforming a hole in the silicon oxide insulating layer, the siliconnitride film is left. Hence, it is possible to avoid the short betweenthe gate electrode and the aluminum electrode formed on the contact holeto the source or the drain. Then, another contact hole for connecting anelectrode for inputting or outputting a signal to or from the gateelectrode is formed by another dry etching process rather than theforegoing etching process. Afterwards, by means of the sputteringdevice, tungsten is deposited to a thickness of 50 nm and then atungsten film having a thickness of 500 nm is formed by means of the CVDmethod. The tungsten layer formed by this sputtering method is used as acohesion layer, because the tungsten film formed by the CVD method haspoor coherence with the silicon insulating film on the ground. Next,like each layer, this tungsten layer is applied to the interconnectionpattern 21 by means of the normal photolithography and the normal drayetching techniques.

[0032] On this interconnection layer is deposited a silicon oxide filmof 200 nm by means of the plasma CVD method. This silicon oxide film isserved as a layer 22 for preventing an intruder that leads to loweringreliability of the semiconductor device such as an contaminant. Further,an electrode connecting hole from the outside of the chip is formed at agiven area. Lastly, for removing the damages caused by the foregoingvarious dry etching processes, the annealing in a hydrogen atmosphere isperformed at a temperature of 450° C. and for 30 minutes. This completesthe semiconductor device according to the invention (see FIG. 4E).

[0033] The estimation is performed with respect to the MOS transistorcharacteristic of the stacked dual gate CMOS semiconductor devicecomposed of W/WNx/polycrystalline silicon formed by the foregoingprocesses. In order to make sure of the effect of the invention, the twoprior arts, that is, the polycrystalline silicon dual gate CMOSsemiconductor device formed by the doping separating method with twomasks and the stacked dual gate CMOS semiconductor device ofW/WNx/polycrystalline silicon doped by the same method, were prepared.The formation of the dual gate by the conventional method needs aprocess of forming a mask for selecting an ion-implanting area fordoping an impurity to a amorphous silicon layer twice. These processesare a mask forming process for doping an n-type or p-type impurity intoa specific area after forming the amorphous silicon and another maskforming process for doping an opposite type impurity on the remainingamorphous silicon area as covering the pre-doped area with a maskmaterial. The formation of the mask materials on the specific area needsthe photolithography and the dry etching processes and the accessoryprocess for removing the mask materials.

[0034] The estimation is performed with respect to the transistorcharacteristic of the present embodiment and the dual gate CMOSsemiconductor device created for the comparison.

[0035] As a representative example, the dependency on the impurityconcentration was obtained. The impurity introduced into the channelarea is intended for adjusting the threshold voltage of the n-channeland the p-channel MOS transistors whose gate electrode working dimensionis 0.25 μm. As shown in FIG. 5, the threshold voltage of the MOStransistor formed by this embodiment indicates the almost samedependency on the impurity concentration of the channel area as thecharacteristic of the stacked dual gate CMOS semiconductor device formedby the conventional method. In particular, these threshold voltages arein proportion to the concentration of the impurity contained in thechannel area. Hence, the leakage of boron to the substrate is negligiblein the operation of the circuit. On the other hand, for thepolycrystalline silicon gate CMOS with no stack of W/WNx, the thresholdvoltage on the substrate surface of the p-channel MOS transistor isshifted from an estimated value by 0.5 V to 1.0 V. In an extreme case,boron is so much leaked that the n-type substrate surface may be changedinto the p-type one. Hence, no normal MOS transistor characteristiccould be obtained. The n-channel MOS transistor indicates the estimateddependency on the concentration of the channel area in the substrateeven if it uses any type of gate electrode. It is understood from theseresults that the dual gate CMOS formed by the present invention allowsthe n-channel and the p-channel MOS transistors to operate normally. Theother transistor basic characteristics such as channel conductance anddriving current do not indicate any defect in an element characteristiccaused by dually doping the n-type and p-type impurities onto theamorphous silicon layer.

[0036] It is understood from the present embodiment that the dual gateCMOS does not show any defective transistor characteristic. As shown inFIG. 1, however, the n-type and the p-type junctions are formed in thethickness direction of the polycrystalline silicon layer of the stackedgate electrode as shown in FIG. 1. The depletion layer is formed on theinterface between the junctions, so that it may disadvantageously serveto be non-conductive. Then, the relationship between the depletion widthand the applied voltage/concentration of the substrate and therelationship between the breakdown voltage of the pn junction and theconcentration of the substrate were obtained by calculation. As shown inFIGS. 6A and 6B, if the concentration of the impurity is about5×10¹⁸/cm³ or more, the depletion width is 1 nm or lower and thebreakdown voltage of the junction is 0.1 V or lower as well. As shown inFIG. 1, since the concentration gradient of boron is acute, the junctionindicates the stepwise distribution of the impurity. On this interface,both of the n-type and the p-type impurities reach as high aconcentration as 1×10²⁰/cm³ or more. It means that no defectiveconduction caused by the formation of the depletion layer is broughtabout. In order to check the studied result, a ring oscillator arrangedto use the dual gate CMOS were produced, in which a signal delay timefor one step of the inverter was estimated. As a result, the signaldelay time of the device formed according to the invention indicates thesame value as the prior art in which the impurities are separately dopedinto the polycrystalline silicon. Concretely, for the MOS transistorwhose gate electrode working dimension is 0.14 μm, the signal delay timeper one step was 20 picoseconds.

[0037] Further, the estimation is performed with respect to the foreignparticle appearing in the doping-related process in the presentinvention and the semiconductor device in which the conventionalimpurity doping method is applied to the polycrystalline silicon layer.For the conventional method, 100 to 200 foreign particles, each of whichis 0.3 μm or more, were detected per an 8-inch wafer, while for thepresent invention, 10 to 80 foreign particles were detected. This isbecause the number of steps for doping the impurities is reduced from 16to 8. In particular, if boron is doped in the formation of the siliconfilm by introducing a boron hydrogen compound gas in the CVD process forforming the amorphous silicon, the foreign particles were reduced morethan those caused by doping boron with the ion-implantation method afterforming the film, concretely, the number of those foreign particles werereduced to 10 to 40.

[0038] The reduction of the foreign particles and the manufacturingsteps in this embodiment serves to improve the manufacturing yield ofthe semiconductor device more than the conventional method by about 5%.The reduction of the manufacturing steps and the improvement of theyield may lead to reduction of the manufacturing cost by severalpercents though it does not apply to any type of semiconductor device.It is assured that the boron segregation onto the WNx interface by theannealing in the hydrogen atmosphere with vapor added thereto leads tosuppressing the shift of the threshold voltage-of the MOS transistorcaused by the boron leakage rather than the conventional polycrystallinesilicon gate. (The hydrogen contained in the silicon nitride film forgate working mask and the gate side wall silicon nitride film for LDDcauses the p+ polycrystalline silicon gate. MOS transistor to invert thetype of the channel area substrate surface into the p type, therebybeing unable to indicate the normal transistor characteristic.) On theother hand, it is understood that the present invention makescontribution to suppressing the fluctuation into 50 mV to 300 mV ratherthan the case where no boron is leaked. It is considered that this sortof fluctuation does not have an adverse effect on the circuitperformance if a device and a circuit design are a little devised.

Second Embodiment

[0039] The dual gate CMOS semiconductor device was manufactured in thealmost same process as the first embodiment. In this embodiment,however, the doping of impurity into the amorphous silicon was performedin the following procedure, which is the same as that of the firstembodiment except the doping process.

[0040] After forming the gate insulating film 5, the amorphous silicon 6was deposited. In the similar method to the first embodiment, thephosphorus ions were implanted on the portion where no mask material 7is covered at 10 keV and in the range of 1×10¹⁴/cm² and 1×10¹⁵/cm² to5×10¹⁵/cm² or more. After removing this mask material, boron wasimplanted on the overall surface of the wafer at 3 keV and in the rangeof 2×10¹⁸/cm³ for doping the boron. As to the boron doping method, inaddition to the ion implanting method, another method was studied suchas an annealing in the state of flowing gas containing boron hydrocarbonand a solid source method for heating boron nitride for doping boron.

[0041] In this embodiment, phosphorus was doped only at the locationwhere the gate of the n-channel MOS transistor is to be formed. Then,boron was doped. It is found that the doped phosphorus is effective insuppressing leakage of the boron doped later onto the substrate throughthe annealing.

[0042] Like the first embodiment, it is assured that the annealing inthe hydrogen atmosphere containing vapor is performed for suppressingleakage of boron. In this embodiment, the annealing in the hydrogenatmosphere with no vapor serves to suppress leakage of boron resultingfrom the phosphorus. That is, the p-channel MOS transistor composed ofamorphous silicon with no phosphorus causes the type of the channel areato be inverted into the p-type because of too much leaked boron, therebybeing unable to operate properly. On the other hand, the n-channel MOStransistor composed of amorphous silicon with phosphorus doped thereinhas few leaked boron. The shift of its threshold voltage against thethreshold voltage of the transistor with no boron leaked is so small as0.1 to 0.7 V. It means that the n-channel MOS transistor indicates thenormal MOS transistor characteristic. This embodiment, therefore,enables to expand the thermal budget against the leaked boron furtherthan the first embodiment.

[0043] In a case that the concentration of phosphorus ion-implanted ontothe amorphous silicon is 1×10¹⁴/cm², the concentration of phosphorussegregated at the gate dielectric film interface is about 5×10¹⁹/cm².Hence, the n-type impurity concentration on the interface with the gateinsulating film is smaller than the p-type impurity, so that thedepletion layer may be formed in the silicon layer of the gate electrodein some applied voltages. Hence, it does not indicate the characteristicestimated for the n-channel MOS transistor having a gate oxide film witha thickness of 3 nm. If the concentration of phosphorus is 1×10¹⁵/cm² ormore, the normal n-channel MOS transistor characteristic was obtained.

Third Embodiment

[0044] In this embodiment, in addition to the doping of phosphorusimpurity in the foregoing embodiment, arsenic ions of 2×10¹⁵/cm² wereimplanted at such an energy as a projection range into the middle of theamorphous or polycrystalline silicon layer. The distribution of theimpurity in the depth direction of the silicon layer of the n-channelgate electrode of the stacked dual gate CMOS semiconductor devicemanufactured as above is illustrated in FIG. 7. It is obvious from FIG.7 that the n-type impurity concentration at the n+/p+ junction formedwithin the polycrystalline silicon layer has a higher concentration thanthose of the first and the second embodiments. Hence, the resistance atthe junction may be reduced further. In place of arsenic, antimony maybe used for offering the same effect.

[0045] In the first and the second embodiments, as the n-type impurityto be doped, in place of phosphorus, arsenic or antimony may be used forachieving the object of the invention. In actual, however, the annealingof 700° C. or more is required for segregating these impurities on theinterface of the gate insulating film.

[0046] Further, in the foregoing embodiments, the boron ions are usedfor doping boron by means of the ion implanting method. Alternatively,the compound ions such as boron fluoride may be used for doping boron.

[0047] According to the foregoing embodiments, in the dual gate CMOSsemiconductor device having a gate electrode composed of p-typepolycrystalline silicon and n-type polycrystalline silicon, just onemask is required for dividing the areas where the n-type and the p-typeimpurities are to be doped. As compared with the conventionalsemiconductor device formed by two masks, the semiconductor devices ofthese embodiments enable to reduce the number of the manufacturingsteps, which leads to reduction of the manufacturing period, improvementof the yield, and reduction of the manufacturing cost of thesemiconductor device. Moreover, the present invention is characterizedby the stacked gate electrode structure and the segregation of boron onthe metal nitride interface. This segregation effect serves to reducethe leaked boron.

What is claimed is: 1: A method of manufacturing a semiconductor devicehaving an NMOS and a PMOS, comprising: forming a gate insulating film ona semiconductor substrate; forming a semiconductor film on said gateinsulating film; forming a mask for covering an area where said PMOS isformed, introducing phosphorus onto a corresponding portion of saidsemiconductor film to an area where said NMOS is formed, and performingan ion implantation of arsenic or antimony at such an energy as aprojection range into the middle of said semiconductor film to said areawhere said NMOS is formed; removing said mask and introducing boron ontothe corresponding portion of said semiconductor film to the area wheresaid NMOS is formed, and the corresponding portion of said semiconductorfilm to the area where said PMOS is formed; forming a metallic nitridefilm on said semiconductor film on the areas where said NMOS and saidPMOS are formed; working a stacked film containing said semiconductorfilm and said metallic nitride film to form each gate electrode of saidNMOS and said PMOS; and performing an annealing in a nitrogen atmosphereor in a hydrogen atmosphere with water vapor added thereto. 2: A methodas claimed in claim 1, wherein, during annealing, boron is segregated onan interface of said metallic nitride film and said semiconductor film,and said phosphorus is segregated on an interface of said gateinsulating film and said semiconductor film. 3: A method as claimed inclaim 1, wherein said semiconductor film formed on said gate insulatingfilm is formed as an amorphous silicon, and said amorphous silicon ischanged into polycrystalline silicon after said annealing. 4: A methodas claimed in claim 1, further comprising: forming a metallic film onsaid metallic nitride film prior to working said stacked film, and saidstacked film containing said semiconductor film, said metallic nitridefilm and said metallic film. 5: A method as claimed in claim 1, whereinsaid gate insulating film is a silicon oxynitride film. 6: A method ofas claimed in claim 1, wherein said boron is introduced by ionimplanting. 7: A method of manufacturing a semiconductor device havingan NMOS and a PMOS, comprising: a first step of forming a gateinsulating film on a semiconductor substrate; a second step of formingan amorphous semiconductor film on said gate insulating film; a thirdstep of forming a mask for covering an area where said PMOS is formed,and introducing phosphorus onto the corresponding portion of saidsemiconductor film to an area where said NMOS is formed, and performingan ion implantation of arsenic or antimony at such an energy as aprojection range into the middle of said semiconductor film to said areawhere said NMOS is formed; a fourth step of removing said mask andintroducing boron onto the corresponding portion of said semiconductorfilm to the area where said NMOS is formed, and the correspondingportion of said semiconductor film to the area where said PMOS isformed; a fifth step of forming a metallic nitride film on saidsemiconductor film on the areas where said NMOS and said PMOS areformed; a sixth step of working a stacked film containing saidsemiconductor film and said metallic nitride film to form each gateelectrode of said NMOS and said PMOS; and a seventh step of performingan annealing in a nitrogen atmosphere or in a hydrogen atmosphere withwater vapor added therto, and making said amposphous silicon film intopolycrystalline silicon film. 8: A method as claimed in claim 7,wherein, at said seventh step, boron is segregated on an interface ofsaid metallic nitride film and semiconductor film, and said phosphorusis segregated on an interface of said gate insulating film andsemiconductor film. 9: A method as claimed in claim 7, furthercomprising an eighth step of forming a metallic film on said metallicnitride film between said fifth and sixth steps, and wherein at saidsixth step, a gate electrode is composed of a stacked film containingsaid semiconductor film, said metallic nitride film and said metallicfilm. 10: A method as claimed in claim 7, wherein said gate insulatingfilm is a silicon oxynitride film. 11: A method of manufacturing asemiconductor device having an NMOS and a PMOS, comprising: a first stepof forming a gate insulating film on a semiconductor substrate; a secondstep of forming a semiconductor film on said gate insulating film; athird step of forming a mask for covering an area where said PMOS isformed, and introducing phosphorus onto the corresponding portion ofsaid semiconductor film to an area where said NMOS is formed, andperforming an ion implantation of arsenic or antimony at such an energyas a projection range into the middle of said semiconductor film to saidarea where said NMOS is formed; a fourth step of removing said mask andintroducing boron onto the corresponding portion of said semiconductorfilm to the area where said NMOS is formed and the corresponding portionof said semiconductor film to the area where said PMOS is formed; afifth step of forming a metallic nitride film on said semiconductor filmon the areas where said NMOS and said PMOS are formed; a sixth step offorming a metallic film on said metallic nitride film on the areas wheresaid NMOS and said PMOS are formed; a seventh step of working a stackedfilm containing said semiconductor film, metallic nitride film and saidmetallic film for forming each gate electrode of said NMOS and saidPMOS; and an eighth step of performing an annealing in a nitrogenatmosphere or in a hydrogen atmosphere with water vapor added thereto.12: A method as claimed in claim 11, wherein, at said eighth step, boronis segregated on an interface of said metallic nitride film andsemiconductor film, and said phosphorus is segregated on an interface ofsaid gate insulating film and semiconductor film. 13: A method asclaimed in claim 11, wherein said semiconductor film formed on said gateinsulating film is formed as an amorphous silicon at said second step,and said amorphous silicon is changed into polycrystalline silicon aftersaid annealing at said eighth step. 14: A method as claimed in claim 11,wherein said gate insulating film is a silicon oxynitride film. 15: Amethod as claimed in claim 11, wherein, at said fourth step, said boronis introduced by ion implanting.